Electrostatic precipitators (“ESPs”) are commonly deployed in industrial applications to remove solid particles from gas flows by charging the particles and causing them to precipitate out of the gas flow. ESPs are useful in industrial and power generation applications to reduce pollution by collecting filterable dust or condensable particulate present in gasses. For example, ESPs are commonly used in fossil fuel power plants, oil and petrochemical refineries, cement plants, paper mills, various incinerators, industrial boilers, metallurgical processes, and other heavy industries to remove particulates from gas streams.
While there are multiple ESP geometries, discussed in further detail below, all ESPs have two primary components: a series of collecting electrodes and a series of discharge electrodes. FIG. 1 depicts a typical prior art configuration of an ESP 10 used in power generation or major industrial applications. A number of large metallic collecting plates 15 are hung vertically and supported by at least two support members 16. The plates 15 are spaced a set distance apart, with the spacing determined by the type of gas and particulates being cleaned. Typically, the plates 15 are anywhere from 9 to 16 inches apart. The plates 15 may be quite large, in some cases having heights exceeding 30 feet. Depending on the amount of particulate to be removed, additional plates 15 may be aligned behind the first row or field of plates 15 to create additional electric fields relative to the direction of gas flow. The collecting plates 15 are electrically grounded.
Between each pair of plates 15 is at least one discharge electrode wire or assembly 20. Typically, there are multiple discharge electrode assemblies 20. Where rigid discharge electrodes are used, as in the embodiment of FIG. 1, each discharge electrode assembly 20 carries multiple discharge electrode points 30. The discharge electrode assembly 20 may be a weighted wire or pipe and spike made of metal or other highly conductive material that carries a negative charge at a voltage above that necessary to achieve corona onset. A typical ESP may have thousands of discharge electrodes. When corona onset occurs (normally about 25 kV for 9″ gas passes), the gas around a discharge electrode and the particulates contained within it becomes ionized. The electrostatic field established between the discharge electrodes and collecting plates directs the negatively charged particles onto the grounded collecting plates.
FIGS. 2A, 2B, and 2C depict a typical prior art configuration for a discharge electrode assembly 20 known as a pipe-and-spike array. A metal pipe 25 passes vertically and halfway between two collecting plates 15 (as shown in FIG. 1). Each discharge electrode point 30 is a spike 31 arrayed horizontally about the pipe 25. The spike 31 has a base 33 that is welded or otherwise secured to the pipe 25. The body 34 of the spike extends out to a end or tip 32. In some ESPs, a single spike 31 is directed in the upstream and downstream direction of the gas flow, such that each spike is parallel to the collecting plates 15 surrounding it, as depicted in FIG. 2C.
In other configurations, two spikes 31 are directed upstream and two spikes 31 are directed downstream. Each pair of spikes 31 may form a “V,” with each spike 31 directed slightly toward one of the two collecting plates 15. In the “V-spike” configuration, depicted in FIG. 2B as a cross-section, each spike 31 carries half the designed current capacity as compared to the single-spike configuration. Multiple sets of discharge electrode spikes 30 are spaced along the length of the metal pipe 25, such that the entire cross-sectional area of gas 40 flowing past a pipe 25 can be ionized, carry charging current, and be scrubbed of particulates 41. One set of spikes 31 is directed upstream, and the second set of spikes 31 is directed downstream. The size and angle of the spikes 31 is dependent upon the ESP's application and the gases 40 and particulates 41 composing the gas flow. For example, in an ESP for scrubbing gas 40 produced by an oil- or coal-fired boiler, the spikes 31 will be approximately 3 inches long and have a nominal diameter between ¼ inch and 3/16 inch. The base 33 of each spike 31 is welded to the pipe 25. The end 32 of the spike 31 is a sharpened metallic point
Wire may also be used for a discharge electrode 30 in place of spikes. The wire may be round, square, twisted, barbed, or in other configurations. Round wire of 0.109″diameter is most common.
Other ESP configurations are also well-known and practiced to meet various design constraints. For example, in a vertical flow ESP, four collecting plates form a vertical, rectangular passage through which the gas flows. In this configuration the discharge electrode assembly has a single pipe dropped through the center of the vertical passage. Multiple spikes are arranged about the metal pipe at set distances. In this configuration, known as a “rod-and-star” array, the spike array for each discharge electrode is perpendicular to the gas flow. Multiple spikes may be arranged about a given point of the pipe.
As depicted in FIG. 3, during operation of a typical ESP, particulate-laden gas 40 is directed through the inlet region 11, passes between the spaced collecting plates, known as gas passes 15, and then exits through the outlet region 12. The arrows represent the direction of gas flow, and the black dots represent the flow of electrons through the circuit. The discharge electrode assembly 20 is charged to a potential difference that causes the onset of negative coronal discharge and ionization of the gas 40. The negatively charged discharge electrodes 30 and grounded collecting plates 15 produce an electrostatic field that electrostatically attracts negatively charged particles to the collecting plates 15. During ionization, the gaseous atoms passing near the discharge electrodes 30 become ionized, as electrons associated with the atoms flow freely. Accordingly, the gas 40 becomes conductive. The negative ions in the gas 40 follow the field lines of the electrostatic field and flow toward the nearest collecting plate 15. In so doing, they attach to particulates 41 carried by the gas 40, which become charged and move to the collecting plates 15 as well. As the gas 40 flows through the gas passes 15 and past additional discharge electrodes 20, particulates 41 build up on the plates 15, forming a collected layer of ash that adheres to the plates 15 and is held there by clamping forces due to electrostatic pressure. The charging current incident on the ash layer is conducted through the ash layer to the grounded collection plate 15. Periodically, a rapper raps the collecting plate 15 to loosen the collected ash layer by accelerating the plate. The separated ash layer then drops into a hopper or other collection device and is disposed of.
ESPs often exhibit sparking in the inlet field where particulate-laden gas begins flowing between the discharge and collecting electrodes. Electrostatic theory indicates that sparking occurs when small volumes of relatively clean gas is interposed between a discharge electrode and a collecting electrode. The resulting lack of particulates significantly reduces the space charge effect in this interelectrode space, which otherwise would be a relatively stable concentration of negatively charged particulate entrained in the area between the electrodes. This increases the magnitude of the electrostatic field at the surface of the collecting plate, which leads to a significant local increase in the intensity of the current discharge from the discharge electrode, which promotes spark initiation. Sparking collapses the electrostatic potential applied to the subject precipitator field, resulting in a temporary decrease of gas ionization and particle charging until the spark is quenched and the power supply is again brought up to voltage. This in turn significantly reduces the efficiency of the ESP.
While it is customary to use highly conductive metals to produce the make the and spikes of a discharge electrode assembly, metals are unable to resist the increased flow of current resulting from the increased gradient and strength of the electrostatic field that results in arcing. Most metals and metal alloys have a resistivity between 1-100 10−8 ohm-meters, with very low dependence on temperature.
In addition to sparking caused by the non-uniform current density that results from varying space charge effects, warped collection plates result in a locally reduced distance between the discharge electrode and collection plate. This greatly reduces the allowable voltage that may be impressed on a discharge electrode array or in such a field before sparking is initiated. Warped collection plates result in significantly reduced efficiency and increased sparking.
Another issue in current ESPs concerns the efficiency of ESPs in applications having gas flows with high-resistivity dust and particles. Dust and particles exhibiting a collected layer resistivity in excess of 1*1012 ohm-cm is considered highly resistive and is susceptible to both sparking and a phenomenon known as “back corona.” A back corona occurs when positive ions are generated by electrical breakdown internally within the collected ash layer. These positive ions migrate back towards the negatively charged discharge electrodes and can cause gas-borne particles to become positively charged or neutralized. The result is very high current flow and power dissipation within the ESP field, without proper dust charging or collection.
What is needed, then, is an ESP having individual electrodes capable of reducing sparking by locally limiting current density to a level that is supportable by the collected ash layer without sparking, while maintaining higher overall power supply and voltage and current.